The very first time I got my VO2max measured I was 18 years old: they put a mask with a tube on my face and mouth, tied it to the nape of my neck while having me run on a treadmill, increasing speed and gradient every 3 minutes, until I reached my absolute maximum effort (or so they thought). 

The result of my maximum oxygen consumption was 4.528 L/min, equal to 72.8 ml/kg/min (my weight was 62.2 kg). 

I was told it was a good result, but my impressions back then were that running that test was totally another thing from running in the track or on the road. 

Indeed the gesture of running on a treadmill is really different, and in any case it requires a lot of specific training in order to be able to use exactly 100% of the muscles just like with the natural gesture. 
The mask (or mouthpiece) is always very annoying and hinders the respiratory flow, seriously stressing those muscles involved in respiration. 
Furthermore, the mask is not always perfectly airtight, it hinders movements and stiffens up neck and shoulder muscles. 

In years of experience in measuring the VO2max, I obtained results that differed a lot from each other quite often; discordances that consisted in different effort protocols, but mainly in measuring instruments of diverse concept and accuracy. 

How useful is the VO2max for the athlete? 

Surely this test gives a valuation on the maximum power of the aerobic engine of the subject, but it is a value not always decisive in competitive results. 

Derek Clayton, marathon world record holder at the end of the 60’s, had a lower VO2max value than mine, about 69 ml/kg/min, but he could run the marathon in 2h09’, approximately 20 minutes faster than my best performance ever. 

This simply because he was able to run at an intensity of 90-92% of his VO2max without accumulating relevant concentrations of lactic acid: less than 2.5 mM/L! 
At the same percentage of my VO2max value, my lactate concentration exceeded 8 mM/l. 

In cycling the measuring of VO2max has about the same meaning as in running: it is useful and indicative of the potentiality of the athlete, but once again its correlation with competitive results is not so tight. 
Lance Armstrong for instance was measured a VO2max of 82 ml/kg/min, which is an excellent value, but common to many other professional athletes that obtained far inferior results in their careers. 

Maybe Lance was not on his top form when taking the test, or maybe he could not give 100% out: surely he also finds the mask or mouthpiece very annoying, but, most importantly, pedaling on a cyclo-ergometer or stationary bike is much different than pedaling on the road.